1. Field of the Invention
The present invention relates to optoelectronic oscillators of improved performance providing high RF frequency stability and reduced noise. These find application in a wide variety of fields, wherever signal generation is required at high respective frequencies with reduced noise, such as radar.
2. Background of the Invention
Microwave generators with multi gigahertz frequency, high spectral purity and high RF frequency stability have applications in many areas such as communication, radar and metrology. Different approaches can be used to obtain high spectral purity and high repetition rate signals. Conventional electronic approaches rely on using a high quality factor (Q) resonator in order to get high spectral purity. Generating microwave signals with electro-optical systems have been studied previously [1,2]. The current optoelectronic oscillator (hereinafter “OEO”) design, introduced by Yao and Maleki, use optical fibers as the high Q element, and have attracted great attention due to their extraordinary spectral purity [3,4]. Several different types of OED's are demonstrated successfully [5] such as the coupled optoelectronic oscillator [6], multi loop OEO architectures [7-9], and OEO with photonic filters that use atomic cells [10]. A standard OEO loop is shown in FIG. 1(a). The conventional OEO consists of a seed laser source followed by a modulator. After passing through an optical delay line, the signal is photodetected, amplified and filtered by an RF filter, and then sent back to the modulator to complete the loop. There are some drawbacks of the standard OEO: a high gain RF amplifier is needed in order to compensate the losses in the RF loop, and it is costly to make an ultra narrow bandwidth RF filter which is required when the optical delay line is long. Moreover, the additional loss from the RF filter decreases the cavity Q of the optoelectronic oscillator which results in an increase in the phase noise. Another drawback of the RF filter is the temperature dependency; small fluctuations of the temperature result in fluctuations at the peak position of the resonance, and hence the phase from the RF filter changes. This phase change affects the total round trip time for the microwave signal, thus changing the oscillation frequency.
OED's of the type described find application in a variety of devices and methods. Radar and signal intelligence are standard applications. OEO generated signals may be used as carriers and are important in clock recovery, and in communication broadcasting and receiving. A single loop OEO is described in detail in U.S. Pat. No. 5,723,856 and a multi-loop OEO is shown in U.S. Pat. No. 5,777,778